Interpretive Summary: Tannins are large polyphenolic polymers and are known to bind proteins, limiting their digestibility. However, tannins are also known for having excellent antioxidant potential and therefore may provide important human health benefits. Some sorghum types contain tannins and interest has grown in the use of these sorghums for human health applications. Little is known how tannin content and chemistry varies from across tannin containing sorghum cultivars and from year to year. This research utilized a novel method to compare the variation in sorghum tannins from seven diverse sorghum cultivars grown over 2 years. Year to year and cultivar to cultivar variation was found in this sample set demonstrating that tannin content and composition plays a significant role on influencing tannin functionality. These differences will allow for selections of high tannin sorghums with consideration of the biological activities of the tannins.

Technical Abstract:
Tannins are large polyphenolic polymers and are known to bind proteins, limiting their digestibility. Tannins are also known for having excellent antioxidant potential. To examine the precise impact of tannin content and composition on the biological activities (protein binding, protein digestibility, and antioxidant capacity) studies were conducted on seven tannin containing sorghum cultivars grown in Kansas over two crop years. Each of the 14 samples were decorticated to have the outer 20% of the kernel (bran and testa layer) removed. This bran was then mixed with a base endosperm from a non-tannin sorghum at five levels. This was done to evaluate the impact of the tannin content and composition from each tannin containing cultivar without interference from innate differences in the endosperm of each cultivar. Tannins in the cultivars were analyzed using a modified vanillin-hydrochloric acid method (MV-HCl) to determine the amount of tannin present. The tannin content of the 14 cultivars tested ranged from 2.3 to 67.2 catechin equivalents, this range can be attributed to both environmental and genetic factors. The bran fractions were studied for their impact on protein binding and antioxidant capacity. Protein digestibility by pepsin ranged from 8% to 58% at the highest tannin level addition. Binding ranged from 3.11 to 16.33 mg blue-BSA/g bran. Antioxidant capacity ranged from 81.33 to 1122.54 µmol TE/g bran. High performance size exclusion chromatography (HP-SEC) was performed on the 14 cultivars to provide detail into the molecular size distribution of the tannin polymers and relationship to tannin functionality. The tannin content and composition plays a significant role on influencing tannin functionality. These differences will allow for selections of high tannin sorghums with consideration of the biological activities of the tannins.